The present invention relates to providing a plurality of organic electroluminescent (EL) units to form a cascaded organic electroluminescent device.
While organic electroluminescent devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic electroluminescent device is comprised of an anode for hole injection, a cathode for electron injection, and an organic layer sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier OLEDs are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, xe2x80x9cDouble Injection Electroluminescence in Anthracenexe2x80x9d, RCA Review, 30, 322-334 (1969); and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 xcexcm). Consequently, operating voltages were very high, often  greater than 100V.
More recent OLEDs include an organic medium consisting of extremely thin layers (e.g.  less than 1.0 xcexcm) between the anode and the cathode. Herein, the term xe2x80x9corganic mediumxe2x80x9d encompasses the layers between the anode and cathode electrodes. Reducing the thickness lowered the resistance of the organic layer and has enabled devices that operate much lower voltage. In a basic two-layer OLED structure, described first by Tang et al. U.S. Pat. No. 4,356,429, one organic layer of the organic medium adjacent to the anode is specifically chosen to transport holes, therefore, it is referred to as the hole-transporting layer (HTL), and the other organic layer is specifically chosen to transport electrons, referred to as the electron-transporting layer (ETL). Recombination of the injected holes and electrons within the organic medium results in efficient electroluminescence.
There have also been proposed three-layer OLEDs that contain an organic light-emitting layer (LEL) between the HTL and the ETL, such as that disclosed by Tang et al. xe2x80x9cElectroluminescence of Doped Organic Thin Filmsxe2x80x9d, J. Applied Physics, 65, 3610-3616 (1989). The LEL commonly consists of a host material doped with a guest material. Still further, there has been proposed by Tang et al. in U.S. Pat. No. 4,769,292 a four-layer OLED adding a hole-injecting layer (HIL) between anode and the HTL. These structures have resulted in improved device performance.
Moreover, in order to further improve the performance of the OLEDs, a new kind of OLED structure called stacked OLED, which is fabricated by stacking several individual OLED vertically, has also been proposed. Forrest et al. in U.S. Pat. No. 5,703,436 and Burrows et al. in U.S. Pat. No. 6,274,980 disclosed their stacked OLEDs. In their inventions, the stacked OLEDs are fabricated by vertically stacking several OLEDs, each independently emitting light of a different color or of the same color. Using their stacked OLED structure can make full color emission devices with higher integrated density in the display, but each OLED needs a separate power source. In an alternative design, Jones et al. in U.S. Pat. No. 6,337,492 proposed a stacked OLED structure by vertically stacking several OLED without individually addressing each OLED in the stack. Jones et al. believe that their stacked structure could increase the luminance output and operational lifetime.
The aforementioned stacked OLEDs use individual OLEDs (anode/organic medium/cathode) as building blocks to fabricate the stacked OLEDs. The complex architecture in these designs presents serious fabrication problems. It is difficult to achieve high optical transparency in the visible light range due to the presence of electrodes internal to the stack (intra-electrodes). This reduces the overall device efficiency.
It is an object of the present invention to make a cascaded OLED with improved overall operational stability.
It is another object of the present invention to make a cascaded OLED with improved voltage stability.
These objects are achieved by a cascaded organic electroluminescent device comprising:
a) an anode;
b) a cathode;
c) a plurality of organic electroluminescent units disposed between the anode and the cathode, wherein the organic electroluminescent units comprise at least a hole-transporting layer and an electron-transporting layer; and
d) a connecting unit disposed between each adjacent organic electroluminescent unit, wherein the connecting unit comprises, in sequence, an n-type doped organic layer, an interfacial layer, and a p-type doped organic layer, wherein the interfacial layer prevents diffusion or reaction between the n-type doped organic layer and the p-type doped organic layer.
An advantage of the present invention is that it enables a cascaded OLED to function without requiring intra-electrodes, thereby lowering optical losses.
Another advantage of the present invention is that the cascaded OLED has very stable driving voltage during operation.
Another advantage of the present invention is that the cascaded OLED provides significantly improved luminance efficiency as measured in cd/A compared to the conventional non-cascaded OLED device.
Another advantage of the present invention is that the cascaded OLED has an increased brightness if operated under the same current as that of the conventional OLED.
Another advantage is that the cascaded OLED has an increased lifetime if operated under the same brightness as that of the conventional OLED.
Another advantage of the present invention is that the cascaded OLED has a decreased driving voltage and an increased optical output compared to the prior arts of stacked OLEDs.
Another advantage of the present invention is that the cascaded OLED can be operated with a single voltage source with only two electrical bus conductors connecting the device to an external circuit. Thus its device architecture is significantly less complex than those reported in the prior art and is therefore much easier and less costly to fabricate.
Another advantage of the present invention is that the cascaded OLED can have a new way to adjust the emission color of the device by mixing appropriate organic electroluminescent units with different color emissions.
Another advantage of the present invention is that high efficiency white electroluminescence can be produced.
Another advantage of the present invention is that the cascaded OLED can be effectively used in a lamp.